Traffic and seasonal dependence of the light absorption coefficient in Santiago de Chile

We have designed and built a compact, low-cost automated system to measure the optical absorption coefficient of air. Because most of the light absorp tion is due to black carbon, this method is a direct measure of the amount of black carbon in the atmosphere. The equipment was used to measure absorp tion over a period of one year in a central area of Santiago. Our results s how a strong correlation with the daily traffic pattern. The highest value of the absorption coefficient during most of the year occurs during the mor ning rush hour (0700-0800), and the lowest value either early in the mornin g (0300-0500) or in the afternoon (1400-1700). The absorption coefficient a lso shows a strong dependence with the season of the year, with values 10-2 0 times higher in winter than in summer. The data show that, during most of the year, the amount of black carbon present in the atmosphere is due to t raffic. At night, during winter, the high concentration of black carbon is due to the temperature inversion effect. (C) 2000 Optical Society of Americ a OCIS codes: 010.1120, 010.1320, 010.1100.